This invention relates to flow control valves and, more particularly, to a needle valve having a non-rotating tapered needle that moves axially along its main axis within concentric cylindrical bores to control the flow of a fluid.
Currently, needle valves are used in a fairly wide variety of applications where precision and accuracy are required in controlling the flow of a fluid. One of such uses is in the area of medical anesthesia machines, such as is described in Braatz U.S. Pat. No. 4,226,573. In such present needle valves, the needle is a tapered member that is rotated by the user and such rotation causes the needle to move with respect to its seat by virtue of a threaded engagement of the needle within a housing. As the needle moves with respect to the seat, obviously, the flow of the fluid is controlled.
One difficulty with such needle valves, however, is in achieving precise concentricity of the needle with respect to its seat which is a torroidal (donut shaped) opening through which the fluid passes. Since conventional needles rotate as they are moved axially with respect to their seats, if the needle is out of round, a slight turn could cause flow to change in an unpredictable manner and an intended decrease of flow could actually turn out to increase the flow through the valve.
The out of round could be created by a needle that is slightly bent or, alternately, by some minute misalignment between the longitudinal axis of the threaded engagement between the valve housing and the needle and/or the longitudinal axis of the cylindrical seat. In any case, the out of round difficulty raises problems with accurate control of the fluid. Correction of the problem can require considerable expense in precise machining, inspection and correction of misalignments during the manufacturing process.
One solution to the problem was suggested in Braatz U.S. Pat. No. 4,575,043 where the needle was deliberately bent so that it physically touched the valve seat at all times in the control of the flow. As such, the profile of the passage through which the fluid passes remains the same, however, the Braatz needle valve utilized a long flexible needle and contact with internal surface of the valve seat was a rather light contact. In the needle valves of the type herein concerned, however, the needles are more rigid and a continual wiping of the internal surface of the valve seat over time can cause wear of the seat and enlarge the opening. Accordingly, for the same setting of the needle, the amount of flow through the present type of needle valve could increase with time.
The problem is, therefore, that the manufacture of a needle to near perfect concentricity is costly and very difficult. Most needles, even after the most careful manufacture, require individual quality testing and an additional special straightening operation. No matter how careful the manufacturing process, therefore, most of the needles need straightening.
Also, as discussed, the current needle valves have spindles affixed to the needle itself and which are threadedly engaged within a needle housing. The spindle also must be perfectly straight and its alignment with the center line of the valve seat and housing extremely precise or the needle is affected and again can be off center and suffer from the aforementioned problems.